Amino groups in the backbone and side chains of a protein can be non-enzymatically bound to a reducing end-group of a reducing sugar, such as glucose, to form an amadori compound, i.e. a glycated protein. In the blood, hemoglobin is glycated to form glycated hemoglobin (glycohemoglobin; HbA1c). The ratio of HbA1c to hemoglobin in patients with diabetes mellitus is higher than that in healthy persons, and the blood level of HbA1c reflects a blood glucose level over a period of past several weeks. Thus, the blood level of HbA1c is very important in diagnosis of diabetes mellitus and as an indicator of blood glucose level control in patients suffering from diabetes mellitus.
Fructosamine oxidases that act upon amadori compounds have been isolated from various species. It has been suggested that glycated albumin, HbA1c, and other glycated proteins and fructosamines can be assayed by the use of fructosamine oxidase (Japanese Patent Public Disclosure No. 61-268178, No. 61-280297, No. 03-155780, No. 05-192193, No. 07-289253, and No. 08-154672; Agric. Biol. Chem., 53(1), 103–110, 1989; Agric. Biol. Chem., 55(2), 333–338, 1991; J. Biol. Chem., 269(44), 27297–27302, 1994; Appl. Environ. Microbiol., 61(12), 4487–4489, 1995; Biosci. Biotech. Biochem., 59(3), 487–491, 1995; J. Biol. Chem., 270(1), 218–224, 1995; J. Biol. Chem., 271(51), 32803–32809, 1996; J. Biol. Chem., 272(6), 3437–3443, 1997); Electrochemistry, 68(11), 869–871, 2000; and Marine Biotechnology, 3, 126–132, Sep. 5, 2001).
As fructosylamine oxidase is a protein, it is desirable to improve the stability of the enzyme.